I have a job interview in English next month. [Alternative: I have a job interview next month which will be conducted in English.]

This is less ambiguous. "An English job interview" could mean "an interview for an English job"; "English job", in turn, could mean a job in England, a job working for a British company, or a job teaching English.

So I am not as good at speaking English as I am at writing. [Alternative: I can't speak English as well as I can write it.]

"As well as" in the phrase "as well as writing" means "in addition to"(も、に加えて), so what you wrote means that you're not good at speaking English and you are also not good at writing English.I think you intended to make a comparison; I gave two ways of saying what I think you probably meant.It's confusing, because "as well as" does indicate a comparison in the alternative sentence.

There was no mistake in the first sentence. In context, most people would understand what you wrote correctly. I pointed out the ambiguity so that you would be aware of it in the future. In some situations, it might be necessary to be more clear.

The problem in the third sentence is a tricky one, since the meaning of "as well as" depends on the grammar of the rest of the sentence. Please don't worry about having made this mistake.

By the way, what you are calling a "diary" should be called "a diary entry" (or simply "an entry") in English. "Diary" and "journal" both mean a book (or an electronic equivalent of a book) in which pieces of personal writing are written. Each individual piece of writing is called an "entry". This is a common mistake, since in Japanese, you use the same word 「日記」 for both. Chinese speakers have the same problem.